The present invention relates to a direct-to-plate lithographic printing method wherein a substrate is coated in an off-press coating apparatus, exposed either on- or off-press and then recycled in an off-press cleaning apparatus.
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
In recent years the so-called computer-to-plate method has gained a lot of interest. This method, also called direct-to-plate method, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. In the field of such computer-to-plate methods the following improvements are being studied presently:
(i) On-press imaging. A special type of a computer-to-plate process involves the exposure of a plate precursor while being mounted on a plate cylinder of a printing press by means of an image-setter that is integrated in the press. This method may be called xe2x80x98computer-to-pressxe2x80x99 and printing presses with an integrated plate-setter are sometimes called digital presses. A review of digital presses is given in the Proceedings of the Imaging Science and Technology""s 1997 International Conference on Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394 and EP-A 774 364. Typical plate materials used in computer-to-press methods are based on ablation. A problem associated with ablative plates is the generation of debris which is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the integrated image-setter. Other methods require wet processing with chemicals which may damage or contaminate the electronics and optics of the integrated image-setter and other devices of the press.
(ii) On-press coating. Whereas a plate precursor normally consists of a sheet-like support and one or more functional coatings, computer-to-press methods have been described, e.g. in GB1546532, wherein a composition, which is capable to form a lithographic surface upon image-wise exposure and optional processing, is provided directly on the surface of a plate cylinder of the press. EP-A 101 266 describes the coating of a hydrophobic layer directly on the hydrophilic surface of a plate cylinder. After removal of the non-printing areas by ablation, a master is obtained. However, ablation should be avoided in computer-to-press methods, as discussed above. U.S. Pat. No. 5,713,287 describes a computer-to-press method wherein a so-called switchable polymer such as tetrahydro-pyranyl methylmethacrylate is applied directly on the surface of a plate cylinder. The switchable polymer is converted from a first water-sensitive property to an opposite water-sensitive property by image-wise exposure. The latter method requires a curing step and the polymers are quite expensive because they are thermally unstable and therefore difficult to synthesize.
(iii) Thermal imaging. Most of the computer-to-press methods referred to above use so-called thermal or heat-mode materials, i.e. plate precursors or on-press coatable compositions which comprise a compound that converts absorbed light into heat. The heat which is generated on image-wise exposure triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex, and after optional processing, a lithographic image is obtained.
(iv) The development of functional coatings which require no wet processing or may be processed with plain water, ink or fountain is another major trend in plate-making. Such materials are especially desired in computer-to-press methods so as to avoid damage or contamination of the optics and electronics of the integrated image-setter by contact with the processing liquids. WO 90002044, WO 91008108 and EP-A 580 394 disclose such plates, which are, however, all ablative plates having a multi-layer structure which makes them less suitable for on-press coating. A non-ablative plate which can be processed with plain water is described in e.g. EP-A 770 497 and EP-A 773 112. Such plates also allow on-press processing, either by wiping the exposed plate with water while being mounted on the press or by the ink or fountain solution applied during the first runs of the printing job.
A computer-to-press method that is characterized by most of the above advantages has been disclosed in EP-A 698 488. An oleophilic substance is image-wise transferred from a foil to a rotary press cylinder by melting said substance locally with a laser beam. The strip-shaped transfer foil has a narrow width compared to the cylinder and is translated along a path which is parallel to the axis of the cylinder while being held in close contact with the surface of the cylinder so as to build up a complete image on that surface gradually. As a result, this system is rather slow and requires a long down-time of the printing press, thereby reducing its productivity.
EP-A 802 457 describes an on-press coating method wherein an aqueous liquid, comprising a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is coated on the plate cylinder so as to form a uniform, continuous layer thereon. Upon image-wise exposure, areas of the coated layer are converted into an hydrophobic phase, thereby defining the printing areas of the printing master. The press run can be started immediately after exposure without any additional treatment because the layer is processed by interaction with the fountain and ink that are supplied to the cylinder during the press run. So the wet chemical processing of these materials is xe2x80x98hiddenxe2x80x99 to the user and accomplished during the first runs of the printing press. After the press-run the coating can be removed from the plate cylinder by an on-press cleaning step. Such methods of on-press coating, on-press exposure and on-press cleaning of the master attract attention because, contrary to conventional lithographic printing, they can be carried out without specialized training or experience. Such presses function more or less like a desktop computer printer and require less human intervention than conventional presses.
A problem associated with the on-press coating, exposure and cleaning methods is that the wet coating and cleaning steps involve a risk of damaging or contaminating the optics and electronics of the integrated image-setter. In addition, the method produces an insufficient coating quality, characterized by a low consistency and a high frequency of coating artifacts, because the printing press is a hostile environment to the application of defect-free coatings due to paper dust, ink misting, and temperature or humidity variations. The quality of the wet-coating step can only be improved by installing a complex and sophisticated coating apparatus on the press, which is difficult to achieve due to space and cost limitations. Finally, during the on-press coating, exposure and cleaning steps, the press is not printing and the press down-time needs to be minimized in order to be economically viable.
It is an object of the present invention to provide a method which is characterized by the advantages of known on-press coating methods but also by a short press down-time and a good coating quality. This object is realized by a method using a printing press that is mechanically coupled to an off-press coating and an off-press cleaning apparatus as defined in the independent claims. A substrate is coated off-press in the coating apparatus, subsequently mechanically transferred to the press, and after the pressrun, the used printing master is mechanically transferred to a cleaning apparatus where the coating is removed from the substrate, which can then be used again in a next cycle of coating, printing and cleaning. The method of the present invention enables a fully-automated workflow of coating, exposure, printing and cleaning wherein the press down-time is minimal and which can be carried out without special skills. The press down-time is minimal because during a pressrun, the imaging material(s) of the next print job can be coated in the coating apparatus and the material(s) of the previous print job can be cleaned in the cleaning apparatus while the press is printing. By using an optional stacking apparatus between the coating apparatus and the press and/or between the cleaning apparatus and the press, a single coating and/or a single cleaning apparatus can be combined with a multi-color printing press which requires more than one printing master. The exposure step can be carried out on-press, offering the benefit of obtaining a perfect registration of the masters in multi-color presses immediately after exposure, or off-press so as to obtain an even lesser press down-time.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings. Preferred embodiments of the invention are disclosed in the dependent claims.